1. Field of the Invention
This invention relates to the field of ceramic composites which comprise a continuous phase and a discontinuous phase, with the discontinuous phase having the form of elongated fibers. Such materials are generally denoted in the art as fiber reinforced composites. The continuous phase may be ceramic, metal, or less often some other material such as a thermosetting resin or a thermoplastic.
2. Technical Background
In general, the fibrous phase in a fiber reinforced composite is more expensive and stronger than the continuous phase. When this is true, the economically optimum physical properties for any given amount of fibrous phase are achieved when the fibers are separately and uniformly distributed throughout the continuous phase and when the aspect ratio, defined as the length to width ratio of the fibers, is at least fairly high. Theoretically, the highest possible aspect ratio is desirable, but above a certain limit there is little if any practical improvement. Because a very high aspect ratio makes uniform distribution very difficult, a practical compromise is generally made between uniformity of distribution and aspect ratio, so that aspect ratios of 15-50 are generally preferred.
Additional general background is available in two papers by John Milewski, "Problems and Solutions in Using Short Fiber Reinforcements", Session 18-C, and "How to Use Short Fiber Reinforcements Efficiently", Session 18-A&B, both in the Proceedings of the 37th Annual Conference, Reinforced Plastics Composites Institute, The Society of the Plastics Industry, Inc., held Jan. 11-15, 1982. These references are designated herein as Milewski I and Milewski II respectively. Another reference, more specific to ceramics, is John Milewski, "Efficient Use of Whiskers in the Reinforcement of Ceramics", 1 Advanced Ceramic Materials, No. 1, p. 36 (1986). This is designated herein as Milewski III.
Achieving or even approaching the optimum distribution of most ceramic reinforcing fibers is difficult in practice. The process of manufacture of fibers often produces bundles, particular when the fibers are the specially strong type known as whiskers. See Milewski III, p. 36. Even if the manufacturing process does not produce bundled fibers directly, fibers with high aspect ratios tend to clump spontaneously because of mechanical entanglement. Id. The fiber bundles or clumps must be broken up to achieve efficient distribution of the fibers.
Two methods have commonly been used in the prior art both to break up the bundles of fibers and to mix them into a dispersion suitable for molding: ball milling and liquid dispersion. Ball milling has the disadvantage that it is likely to break many of the fibers, thereby reducing the average aspect ratio and leading to an undesirably wide distribution of aspect ratios for the individual fibers. Ball milling may also damage fibers in such a way as to reduce their strength, even if the fibers remain intact. Liquid dispersion requires a solvent and a dispersing aid, and effective solvents and dispersing aids are not always easy to find. Little guidance from theory is available for the choice. Also, the solvent generally must be substantially removed before a suitable coherent green body can be formed from the dispersion. This requires an additional processing step and thus increases costs.
A combination of both common methods is taught by U. S. Pat. No. 4,463,058 of Jul. 31, 1984 to Hood et al. This reference describes deagglomerating silicon carbide fibers by dispersing them in a polar solvent and ball milling the resulting dispersion until it is deagglomerated, adding metal powder either dry of in slurry to the deagglomerated silicon carbide fiber dispersion, removing the bulk of the polar solvent by distillation or drying, and molding and eventually sintering the almost dry dispersion formed by removing most of the polar solvent.
Another methods of dispersion taught in the prior art is ultrasonic dispersion, which is recommended by the Milewski III reference, p. 38. and by P. Becher and G. Wei, "Toughening Behavior in SiC-Whisker-Reinforced Alumina", Communications of the American Ceramic Society, December 1984, p. C-267.